Just being metal doesn't make a box grounded. Self-grounding recepts. have to be installed in a grounded box, or they are not grounded either. To ground the box you are going to have to get a ground wire back to the source ground.
'Grounding' back through dirt don't cut it. You have to get the ground back to the panel ground. Too much resistance in the earth to make for a good equipment ground path.
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